If a anvil is placed by right clicking on a block, the redstone under neath is destroyed, and the anvil takes its place. But if the anvil is placed by right clicking on the redstone the anvil will floot above the redstone
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This isn't a bug, and it will happen with any block. This is because the redstone counts as a block, and if you place something on it, it's just going to rest in the air.